Whisk together the mayonnaise, sour cream, milk, and lemon zest and juice in a medium mixing bowl.
Mix in the rosemary leaves, green onion, garlic, salt, and pepper until well combined.
Refrigerate at least 2 hours or overnight for best results. Store covered in the fridge.
SNAPPY TIPS: You can also make this in a food processor or blender. I suggest adding the lemon juice and zest, garlic, rosemary, scallion, salt, and pepper with the milk then process until the herbs are well chopped. Pour into a bowl and stir in the mayonnaise and sour cream. SNAPPY SUBSTITUTIONS: Instead of milk you could use buttermilk. Instead of mayonnaise and/or sour cream you can try plain unsweetened yogurt or Greek yogurt as well as low fat versions. Results may vary.